The use of ink jet printers for printing information on a recording media is well established. Printers employed for this purpose may be grouped into those that continuously emit a stream of fluid droplets, and those that emit droplets only when corresponding information is to be printed. The former group is generally known as continuous inkjet printers and the latter as drop-on-demand inkjet printers. The general principles of operation of both of these groups of printers are very well recorded. Drop-on-demand inkjet printers have become the predominant type of printer for use in home computing systems, whereas continuous inkjet systems find major application in industrial and professional environments. Typically, continuous inkjet systems produce higher quality images at higher speeds than drop-on-demand systems.
Continuous inkjet systems typically have a print head that incorporates a fluid supply system for fluid and a nozzle plate with one or more nozzles fed by the fluid supply. The fluid is jetted through the nozzle plate to form one or more thread-like streams of fluid from which corresponding streams of droplets are formed. Within each of the streams of droplets, some droplets are selected to be printed on a recording surface, while other droplets are selected not to be printed, and are consequently guttered. A gutter assembly is typically positioned downstream from the nozzle plate in the flight path of the droplets to be guttered.
In order to create the stream of droplets, a droplet generator is associated with the print head. The droplet generator stimulates the stream of fluid within and just beyond the print head, by a variety of mechanisms known in the art, at a frequency that forces continuous streams of fluid to be broken up into a series of droplets at a specific break-off point within the vicinity of the nozzle plate. In the simplest case, this stimulation is carried out at a fixed frequency that is calculated to be optimal for the particular fluid, and which matches a characteristic drop spacing of the fluid jet ejected from the nozzle orifice. The distance between successively formed droplets, S, is related to the jet velocity, v, and the stimulation frequency, f, by the relationship: v=f S. U.S. Pat. No. 3,596,275, issued to Sweet, discloses three types of fixed frequency generation of droplets with a constant velocity and mass for a continuous inkjet recorder. The first technique involves vibrating the nozzle itself. The second technique imposes a pressure variation on the fluid in the nozzle by means of a piezoelectric transducer placed typically within the cavity feeding the nozzle. A third technique involves exciting a fluid jet electrohydrodynamically (EHD) with an EHD droplet stimulation electrode.
Additionally, continuous inkjet systems employed in high quality printing operations typically require small closely spaced nozzles with highly uniform manufacturing tolerances. Fluid forced under pressure through these nozzles typically causes the ejection of small droplets, on the order of a few pico-liters in size, traveling at speeds from 10 to 50 meters per second. These droplets are generated at a rate ranging from tens to many hundreds of kilohertz. Small, closely spaced nozzles, with highly consistent geometry and placement can be constructed using micro-machining technologies such as those found in the semiconductor industry. Typically, nozzle channel plates produced by these techniques are typically made from materials such as silicon and other materials commonly employed in micromachining manufacture (MEMS). Multi-layer combinations of materials can be employed with different functional properties including electrical conductivity. Micro-machining technologies may include etching. Therefore through-holes can be etched in the nozzle plate substrate to produce the nozzles. These etching techniques may include wet chemical, inert plasma or chemically reactive plasma etching processes. The micro-machining methods employed to produce the nozzle channel plates may also be used to produce other structures in the print head. These other structures may include ink feed channels and ink reservoirs. Thus, an array of nozzle channels may be formed by etching through the surface of a substrate into a large recess or reservoir which itself is formed by etching from the other side of the substrate.
FIG. 1 schematically illustrates a prior art conventional electrohydrodynamic (EHD) stimulation means used to excite a jet of conductive fluid into a stream of droplets. Fluid supply 10 contains conductive fluid 12 under pressure which forces ink through nozzle channel 20 in the form of a conductive fluid jet 22. Conductive fluid 12 is grounded or otherwise connected through an electrical pathway. A prior art droplet stimulation electrode 15 is approximately concentric with an exit orifice 21 of nozzle channel 20 as shown in cross-section in FIG. 1A. Droplet stimulation electrode 15 typically includes a conductive electrode structure 13 produced from a variety of conductive materials, including a surface metallization layer, or from one or more layers of a semiconductor substrate doped to achieve certain conductivity levels. Prior art conductive electrode structure 13 is electrically connected to a stimulation signal driver 17 that produces a potential waveform of chosen voltage amplitude, period and functional relationship with respect to time in accordance to a stimulation signal 19. In FIG. 1, an example of a stimulation signal 19 comprises a uni-polar square wave with a 50% duty cycle. The resulting EHD stimulation is a function of the square of field strength created at the surface of the conductive fluid 12 near exit orifice 21. The resulting EHD stimulation induces charge in the conductive fluid jet 22 and creates pressure variations along the jet. Conductive electrode structure 13 is covered by one or more insulating layers 24 which are necessary to isolate droplet stimulation electrode 15 from conductive fluid 12 in order to prevent field collapse, excessive current draw and/or resistive heating of conductive fluid 12. The conductive fluid 12 must be sufficiently conductive to allow charge to move through the fluid from the grounded fluid supply 10 in order to electrohydrodynamically stimulate conductive fluid jet 22 to form droplets that subsequently form at break-off point 26. Since conductive fluids are employed, a non-uniform distribution of charge cannot be supported in the fluid jet column outside of the stimulating electric field. The electrohydrodynamic stimulation effect occurs due to the momentary induction of charge in conductive fluid 12 at nozzle orifice 20 that creates the pressure variation in fluid jet 22. For a correctly chosen frequency of the stimulation signal 19, the perturbation arising from the pressure variations will grow on the conductive fluid jet 22 until break-off occurs at the break-off point 26.
Various means for distinguishing or characterizing printing droplets from non-printing droplets in the continuous stream of droplets have been described in the art. One commonly used practice is that of electrostatic charging and electrostatic deflecting of selected droplets as described in U.S. Pat. No. 1,941,001, issued to Hansell, and U.S. Pat. No. 3,373,437, issued to Sweet et al. In these patents, a charge electrode is positioned adjacent to the break-off point of fluid jet. Charge voltages are applied to this electrode thus generating an electric field in the region where droplets separate from the fluid. The function of the charge electrode is to selectively charge the droplets as they break off from the fluid jet.
Referring back to FIG. 1, a typical prior art electrostatic droplet characterizing means includes charging electrode 30. Conductive fluid 12 is employed such that a current return path exists through the fluid supply 10 (e.g. through grounding). A charge is induced in a specific droplet under the influence of the field generated by charge electrode 30. This droplet charge is locked in on the droplet when it separates from the fluid jet 22. Charging electrode 30 is electrically connected to charge electrode driver 32. The charging electrode 30 is driven by a time varying voltage. The voltage attracts charge through conductive fluid 12 to the end of the fluid stream where it becomes locked-in or captured on charged droplets 34 once they break-off from the jet 22.
A high level of conductivity of fluid 12 is required to effectively charge droplets formed in these prior art systems. Prior art inkjet print heads that employ electrostatic droplet characterizing means typically use conductive fluid 12 conductivities on the order of 5 mS/cm. These conductivity levels permit induction of sufficient charge on charged droplets 34 to allow downstream electrostatic deflection. The conductivity required for droplet charging is typically much greater than that for droplet stimulation. Typically, a conductive fluid suitable for charging can also be stimulated using EHD principles. The selective charging of the droplets in conventional electrostatic prior art inkjet systems allows each droplet to be characterized. That is, the conductive inks permit charges of varying levels and polarities to be selectively induced on the droplets such that they can be characterized for different purposes. Such purposes may include selectively characterizing each of the droplets to be used for printing or to not be used for printing.
Again referring to the prior art system shown in FIG. 1, a potential waveform produced by the charging electrode driver 32 will determine how the formed droplets will be characterized. The potential waveform will determine which of the formed droplets will be selected for printing and which of the formed droplets will not be selected for printing. Droplets in this example are characterized by charging as shown by charged droplets 34 and uncharged droplets 36. Since a specific droplet characterization is dependant upon whether that droplet is printed with or not, the potential waveform will typically be based at least in part on a print-data stream provided by one or more systems controllers (not shown). The print-data stream typically comprises instructions as to which of the specific droplets within the stream of droplets are to be printed with, or not printed with. The potential waveform will therefore vary in accordance with the image content of the specific image to be reproduced.
Additionally, the potential waveform may also be based on methods or schemes employed to improve various printing quality aspects such as the placement accuracy of droplets selected for printing. Guard drop schemes are an example of these methods. Guard drop schemes typically define a regular repeating pattern of specific droplets within the continuous stream of droplets. These specific droplets, which may be selected to print with if required by the print-data stream, are referred to as “print-selectable” droplets. The pattern is additionally arranged such that additional droplets separate the print-selectable droplets. These additional droplets cannot be printed with regardless of the print-data stream and are referred to as “non-print selectable” droplets. This is done so as to minimize unwanted electrostatic field effects between the successive print-selectable droplets. Guard drop schemes may be programmed into one or more systems controllers (not shown) and will therefore alter the potential waveform so as to define the print-selectable droplets. The voltage waveform will therefore characterize printing droplets from non-printing droplets by selectively charging individual droplets within the stream of droplets in accordance with the print data stream and any guard drop scheme that is employed.
Again referring to the prior art system shown in FIG. 1, electrostatic deflection plates 38 placed near the trajectory of the characterized droplets interact with charged droplets 34 by steering them according to their charge and the electric field between the plates. In this example, charged droplets 34 that are deflected by deflection plates 38 are collected on a gutter 40 while uncharged droplets 36 pass through substantially un-deflected and are deposited on a receiver surface 42. In other systems, this situation may be reversed with the deflected charged droplets being deposited on the receiver surface 42. In either case, further complications arise from the fact that the charging electrode driver 32 must be synchronized with stimulation signal driver 17 to ensure that optimum charge levels are transferred to droplets, thus ensuring accurate droplet printing or guttering as the architecture of the recorder may dictate. These synchronization constraints arise as result of charging or characterizing those conductive fluid droplets at a place and time separate from their stimulation. Although prior art electrostatic characterization and deflection systems are advantageous in that they permit large droplet deflection, they have the disadvantage that they have been used primarily only with conductive fluids, thus limiting the applications of these systems.
A wide range of fluid properties is desirable in commercial inkjet applications. Jetted inks may be made with pigments or dyes suspended or dissolved in fluid mediums comprised of oils, solvents, polymers or water. These fluids typically have a large range of physical properties including viscosity, surface tension and conductivity. Some of these fluids are considered to be non-conductive fluids, and thus have insufficient levels of conductivity so as to be employed in continuous inkjet systems that rely on the selective electrostatic charging and deflection of conductive fluid droplets.
Various systems and methods for stimulating a non-conductive fluid medium to form a series of droplets and for characterizing the series of droplets to form “printing” droplets and “non-printing” droplets have been proposed. For example, U.S. Pat. No. 3,949,410, issued to Bassous et al., teaches use of a monolithic structure useful for the EHD stimulation of conductive fluid droplets in a jet stream emitted from a nozzle.
U.S. Pat. No. 6,312,110, issued to Darty, and U.S. Pat. No. 6,154,226, issued to York et al., teach the construction of various inkjet print heads wherein droplets are not stimulated from a stream of non-conductive fluid. Rather, the print heads comprises EHD pumps within the print head nozzles themselves. Droplets are ejected from the fluid supply in a similar fashion to drop-on-demand printers.
U.S. Pat. No. 4,190,844, issued to Taylor, teaches a use of a first pneumatic deflector for deflecting non-printing ink droplets towards a droplet catcher. A second pneumatic deflector either creates an “on-off” basis for line-at-a-time printing, or a continuous basis for character-by-character printing.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., teaches a use of asymmetric heaters to both create and deflect individual droplets formed in a continuous inkjet recorder. Deflection of the droplets occurs by the asymmetrical heating of the jetted stream.
U.S. Pat. No. 4,123,760, issued to Hou, teaches the use of deflection electrodes upstream of a break-off point from which droplets are formed from a corresponding jetted fluid stream. Droplets produced by the stream are steered to different laterally separated printing locations by applying a cyclic differential charging signal to the deflection electrodes. This causes a deflection of the unbroken fluid stream which directs the droplets towards their desired printing positions.
It can be seen that there is a need to provide an apparatus and method of characterizing a non-conductive fluid droplet or droplets formed from a jet of non-conductive fluid.